Pressure to use renewable energy sources is growing. One renewable energy source is the sun. Energy from the sun can be harvested by converting sunlight to electricity using a photovoltaic cell or a photoelectrochemical cell. Both such cells are commonly known as “solar cells.”
A photoelectrochemical cell includes a photoactive semiconductor working electrode and a counter electrode made of either metal or semiconductors. Both of the electrodes are immersed in a suitable electrolyte. The materials used as electrodes must have a suitable optical functionality (absorption of solar energy) as well as a suitable catalytic functionality (for reactions such as water decomposition). There are several types of photoelectrochemical cells, including electrochemical photovoltaic, electrochemical photocatalytic, electrochemical photoelectrolytic cells. In systems using photocatalysis, the “catalyst” is typically either a semiconductor surface or an in-solution metal complex. In addition to producing electricity, photoelectrochemical cells can produce fuels (e.g., hydrogen, etc.).
A basic photovoltaic cell includes an “optically active” region in which electricity is generated and two contacts for extracting that electricity. The optically active region typically comprises abutting layers of n-type semiconductor and p-type semiconductor. A single p-n junction is created at the interface of the layers, thereby creating an electric field.
When sunlight shines on either a photoelectrochemical or a photovoltaic cell, it can be reflected, absorbed, or transmitted. Only the light that is absorbed in the optically active region ultimately generates electricity. More particularly, only photons having an energy that is at least equal to the band gap of the active region can free an electron from (the semiconductor material in) that region. It is those freed electrons, in conjunction with electric field(s) established in the active region, which create an electrical current. In other words, the photovoltaic cell can only use the portion of the sun's spectrum that is above the band gap of the absorbing material; lower-energy photons are not used. The band gap for most photovoltaics is 1.1-1.7 eV, which correlates to wavelengths in the range of about 1100 nanometers (nm) to about 730 nm.
One way to address the issue of the sun's “useful” spectrum is with a multi-junction cell. A multi-junction cell is essentially a stack of individual single-junction cells arranged in order of descending band-gap. Photons below the band gap of the first cell are not absorbed thereby and are transmitted to second cell. The second cell absorbs the higher-energy portion of the remaining solar radiation and remains transparent to photons below its band gap. This selective absorption process continues to the final cell, which has the lowest band-gap. In this way, multi-junction cells convert a greater range of the sun's energy spectrum to electricity than single-junction cells, thus potentially achieving higher total conversion efficiency.
Unfortunately, the increased efficiency of multi-junction photovoltaics comes at a price. Specifically, multi-junction cells require both photocurrent and lattice matching between the various semiconductor layers for high performance. Materials that satisfy these criteria are expensive and require costly deposition processes.
Another approach to improving the efficiency of a solar cell is through spectral conversion. In this process, the solar spectrum is altered to better match the wavelength-dependent conversion efficiency of the solar cell. In spectral up-conversion, two photons each having an energy lower than the band gap are converted to one photon having an energy above the band gap. The higher-energy photons produced by this process are directed back to the solar cell and can then be absorbed, thereby increasing the cell's maximum short-circuit current and, consequently, its efficiency.
Materials suitable for use as an “up-converter” will emit photons having a higher energy than the photons that the material absorbs. A two-step excitation process is required to up-convert the infrared part of the solar spectrum. Materials suitable for this process include lanthanide and transition metal ions, quantum dots, and metal-ligand complexes.
Thus far, only lanthanide up-converters have been used in solar cells. For up-conversion from near infrared to visible wavelengths, the most efficient material known is NaYF4:Yb3+, Er3+, wherein Yb3+ is the sensitizer and Er3+ is the emitter. This material absorbs light at about 980 nm and up-converts it to green and red light (i.e., about 700 to 500 nm). In practice, up-conversion efficiency is low and to date most actual demonstrations have merely served as “proof-of-principle.”                FIGS. 1A through 1E depict five different configurations for solar cells having up-conversion capability:        FIG. 1A depicts cell 100A, wherein active region 102 comprises gallium arsenide and up-conversion region 104 comprises vitroceramics.        FIG. 1B depicts cell 100B, wherein active region 112 comprises crystalline silicon and up-conversion region 114 comprises NaYF4:20% Er3+.        FIG. 1C depicts cell 100C, wherein active region 122 comprises amorphous silicon and up-conversion region 124 comprises NaYF4:18% Yb3+, 2% Er3+.        FIG. 1D depicts cell 100D, which is a dye-sensitized solar cell (“DSSC”) wherein active region 132 comprises titanium dioxide and up-conversion region 134 comprises YAG 3% Yb3+, 0.5% Er3+.        FIG. 1E depicts cell 100E, which is also a dye-sensitized solar cell (“DSSC”) having active region 142 and a combined active/up-conversion region 144.        
Cells 100A through 100D have a discrete, electrically-isolated layer of up-conversion material (i.e., layers 104, 114, 124, and 134), which is distinct from any other functional element of the cell. In cell 100E, the up-conversion material is not electrically isolated; rather, it is mixed with some of the titanium oxide, forming layer 144.
Back reflectors 106 (cells 100A through 100D) reflect all emitted photons back into the cell. As previously indicated, all solar cells have contacts for extracting the electricity formed in the active layer. For example, cell 100A includes metal contacts 108, cell 100C has ZnO contacts 118, and cell 100E incorporates FTO (fluorine-doped tine oxide) contacts 142. In cell 100B, which is a bifacial buried contact solar cell, the electrical contacts (not depicted) are embedded in grooves in active region 112. Cell 100D has transparent front and back contacts (not depicted).
Most prior-art up-converting solar cells include a transparent back electrode between the up-converting region and the active region of the cell (so that light can reach the up-converting region and then be reflected into the active region). This increases the cost of the solar cell, reduces the amount of sub-band gap light that can be absorbed by the up-converting region, and reduces the amount of up-coverted light that can be absorbed in the active region. Some other prior-art up-converting solar cells have both electrical contacts at the front of the cell, which limits the absorption of incident light. Up-converting solar cells are discussed in de Wild, et al., “Upconverter Solar Cells: materials and applications,” Energy Environ. Sci, v4, 4835-4848 (2011); Ghozati et al., “Improved Fill-factor for the Double-Sided Buried-Contact Bifacial Silicon Solar Cell,” Solar Energy Materials and Solar Cells, v51, 121-128 (1998); Liu et al, “Enhancing Near-Infrared Solar Cell Response Using Upconverting Transparent Ceramics,” Solar Energy Materials and Solar Cells, v95, 800-803 (2011); Shan et al., “The Hidden Effects of Particle Shape and Criteria for Evaluating the Upconversion Luminescence of Lanthanide Doped nanophosphors,” J. Phys. Chem., v114, 2452-2461 (2010); and Wang et al., “Rare-Earth Ion Doped Upconversion Materials for Photovoltaic Applications,” Adv. Matl., v23, 2675-2680.
The art would therefore benefit from improvements in the structure and performance of up-conversion materials. For solar cell applications, this would increase the efficiency of such cells and reduce their cost relative to prior-art solar cells.